1 Technical Field of the Invention
The present invention relates generally to a gas concentration measuring apparatus for measuring the concentration of gases which may be employed in an air-fuel ratio control system for automotive vehicles, and more particularly to a gas concentration measuring apparatus designed to measure two kinds of gas components and offset an error in measuring one of the gas components which depends upon the other gas component.
2 Background Art
The air pollution caused by exhaust emissions of automotive internal combustion engines is giving rise to a serious problem at the present day. The exhaust emission control standard regulations have been made more rigorous recently. The burning control of gasoline or diesel engines or use of catalyst are, therefore, being studied to reduce pollutants contained in exhaust gasses. In U.S., OBD-II (On Board Diagnostic-II) requirements prescribe that automotive vehicles have a function of determining whether a catalytic converter is operating normally or not.
As one of systems meeting the OBD-II requirements, a two-O2 sensor monitoring system is proposed which monitors outputs of two O2 sensors mounted upstream and downstream of a catalytic converter, respectively, but it is not designed to detect pollutants directly and cannot determine whether pollutants in exhaust gasses have been reduced or not accurately.
If it becomes possible to measure the concentration of NOx in exhaust gasses for monitoring the burning control and the catalytic converter, the pollutants in the exhaust gasses can be reduced greatly. Specifically, the reduction in pollutants in exhaust emissions of the engine is achieved by controlling the quantity of fuel to be injected into the engine and the EGR rate based on the concentration of NOx contained in the exhaust gasses. Additionally, the determination of deterioration of the catalytic converter is achieved easily by installing a NOx sensor downstream of the catalytic converter.
NOx sensors capable of measuring the concentration of NOx accurately and techniques for mounting such NOx sensors in automotive vehicles are, therefore, being sought.
The effects of air-fuel ratio feedback control may be improved further by monitoring the concentration of O2 contained in the exhaust gasses as well as the concentration of NOx. Specifically, modern air-fuel ratio control for automotive vehicles is required to improve the accuracy of the control and perform lean burn engine control. For meeting these requirements, sensors capable of determining the air-fuel ratio of a mixture supplied to the engine over a wide range are being sought.
U.S. Pat. No. 5,866,799 teaches a NOx sensor designed to reduce the quantity of O2 contained in exhaust gasses first and then measure the concentration of NOx in the exhaust gasses. The measurement of concentration of NOx is achieved by decomposing NOx gas components into oxygen ions and measuring an electric current produced by the flow of the oxygen ions through electrodes. This type of NOx sensor, however, has the drawback in that the part of O2 gas contained in the exhaust gasses entering the sensor reaches the electrodes for measuring the concentration NOx, which causes an error component to be produced in the current indicative of the concentration of NOx. This problem will also be referred to in detail later in description of embodiments of the invention.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a gas concentration measuring apparatus designed to measure two kinds of gas components and offset an error in measuring one of the gas components which depends upon the other gas component.
According to one aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a diffused resistor into which gasses flow, a first cell responsive to application of a voltage to discharge oxygen contained in the gasses outside the gas concentration sensor, producing a first electric current as a function of concentration of the discharged oxygen, and a second cell responsive to application of a voltage to produce a second electric current as a function of concentration of a specified gas component contained in the gasses from which the oxygen is discharged by the first cell; (b) a first current measuring circuit measuring the first electric current flowing through the first cell of the gas concentration sensor; (c) a second current measuring circuit measuring the second electric current flowing through the second cell of the gas concentration sensor; and (d) a correcting circuit correcting the second electric current measured by the second current measuring circuit based on the first electric current measured by the first current measuring circuit to compensate for an oxygen-caused error component of the second electric current which depends upon the concentration of oxygen in the gasses and provide an error-corrected second electric current.
In the preferred mode of the invention, if the first electric current is defined as Ip and the second electric current is defined as Is, the correcting circuit provides the error-corrected second electric current Isf according to the following equation:
Isf=Isxc2x7Kb/(Kaxc2x7Ip+Kb)
where Ka is a structural constant defined by a structure of the gas concentration sensor, and Kb is a correction coefficient defined by sensitivity of the second cell.
The structural constant Ka is determined by a diffusion coefficient, a shape, and a volume of the diffused resistor, and locations of the first and second cell in the gas concentration sensor.
The correcting circuit stores correction data representing a relation between the concentration of oxygen in the gasses and the oxygen-caused error component of the second electric current and monitors the first electric current to determine the error-corrected second electric current based on the correction data.
The correction data is so defined that the concentration of the specified gas component indicated by the second electric current is decreased as the concentration of oxygen indicated by the first electric current increases.
According to the second aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a diffused resistor into which gasses flow, a first cell responsive to application of a voltage to discharge oxygen contained in the gasses outside the gas concentration sensor, producing a first electric current as a function of concentration of the discharged oxygen, and a second cell responsive to application of a voltage to produce a second electric current as a function of concentration of a specified gas component contained in the gasses from which the oxygen is discharged by the first cell; and (b) a correcting circuit correcting the second electric current flowing through the second cell to compensate for a residual oxygen-caused error component contained in the second electric current which depends upon a quantity of oxygen remaining on the second cell without being discharged by the first cell.
In the preferred mode of the invention, an offset current measuring circuit is further provided which measures an offset current flowing through the second cell as a function of the quantity of oxygen remaining on the second cell. The correcting circuit compensates for the residual oxygen-caused error component based on the offset current.
The second cell is so designed as to produce the offset current plus the second electric current in response to the application of the voltage in a first voltage level range and only the offset current in response to the application of the voltage in a second voltage level range different from the first voltage level range. The offset current measuring circuit applies the voltage within the second voltage level range to the second cell to measure the offset current.
The correcting circuit may apply the voltage within the first voltage level range to the second cell to measure the second electric current and apply the voltage within the second voltage level range to the second cell to measure the offset current. The correcting circuit compensates for the residual oxygen-caused error component based on the offset current.
A switching circuit is further provided which switches between a first and a second voltage application mode. In the first voltage application mode, the voltage in the first voltage level range is applied to the second cell. In the second voltage application mode, the voltage in the second voltage level range is applied to the second cell.
An electromotive force measuring circuit may alternatively be provided which measures an electromotive force produced by the second cell as a function of the quantity of oxygen remaining on the second cell. The correcting circuit offsets the residual oxygen-caused error component of the second electric current based on the electromotive force measured by the electromotive force measuring circuit.
The electromotive force measuring circuit includes a switch which is turned on to block communication between the second cell of the gas concentration sensor and a voltage source applying the voltage to the second cell. The electromotive force measuring circuit measures the electromotive force when the switch is turned on.
According to the third aspect of the invention, there is provided a gas concentration measuring apparatus which comprises: (a) a gas concentration sensor including a diffused resistor into which gasses flow, a first cell responsive to application of a voltage to discharge oxygen contained in the gasses outside the gas concentration sensor, producing a first electric current as a function of concentration of the discharged oxygen, and a second cell responsive to application of a voltage to produce a second electric current as a function of concentration of a specified gas component contained in the gasses from which the oxygen is discharged by the first cell; (b) a first current measuring circuit measuring the first electric current flowing through the first cell of the gas concentration sensor; (c) a second current measuring circuit measuring the second electric current flowing through the second cell of the gas concentration sensor; and (d) a correcting circuit performing a first correcting operation and a second correcting operation, the first correcting operation correcting the second electric current measured by the second current measuring circuit based on the first electric current measured by the first current measuring circuit to compensate for an oxygen-caused error component of the second electric current which depends upon the concentration of oxygen in the gasses, the second correcting operation correcting the second electric current to compensate for a residual oxygen-caused error component contained in the second electric current which depends upon a quantity of oxygen remaining on the second cell without being discharged by the first cell.
In the preferred mode of the invention, the correcting circuit selectively performs the first and second correcting operations according to given requirements of the first and second correcting operations.
The first and second current measuring circuits measure the first and second electric currents in a cycle. The correcting circuit performs the first correcting operation in a first cycle shorter than a second cycle in which the second correcting operation is performed.
According to the fourth aspect of the invention, there is provided a gas concentration sensor which comprises: (a) a diffused resistor into which gasses flow; (b) a first cell responsive to application of a voltage to discharge oxygen contained in the gasses outside the gas concentration sensor, producing a first electric current as a function of concentration of the discharged oxygen; and (c) a second cell responsive to application of a voltage to produce a second electric current as a function of concentration of a specified gas component contained in the gasses from which the oxygen is discharged by the first cell, the second cell being so designed as to produce an offset current plus the second electric current in response to the application of the voltage in a first voltage level range and only the offset current in response to the application of the voltage in a second voltage level range different from the first voltage level range.
Each of the first and second cell includes a first electrode exposed to the diffused resistor and a second electrode located away from the diffused resistor. The first electrode of the first and second cells are made of a material which is inactive with respect to the specified gas component.